Light filter



July 7, 1970 R. s. SCHOOLS E'TAL 3,519,355

FIG.1

DISTANCE INVENTORS RODMAN S. SCHOOLS GLENN T. SINCERBOX BY "QM 30k ATTORNEY y 1970 R. s.,scHoo| s :rm. 3,519,355

' LIGHT FILTER Filed 9 2 Sheets-Sheet z FIG.5 36 44 United States Patent 3,519,355 LIGHT FILTER Rodman S. Schools, Poughkeepsie, and Glenn T. Sincerbox, Wappingers Falls, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Dec. 18, 1964, Ser. No. 419,404 Int. Cl. G01!) 9/02 US. Cl. 356-106 Claims ABSTRACT OF THE DISCLOSURE Light filtering apparatus is described in which a transparent film has light reflecting layers periodically spaced from each other. The spacing distances vary in a straight line ratio between two spaced locations of the filter. Such filter apparatus may be employed to determine the unknown frequency of a beam of light or for generating a light beam having a desired frequency.

This invention relates to light filters, and more particularly to a filter having the ability to reflect light which varies continuously in frequency from one end of the filter to the other.

It is well known in Lippmann photography that thin layers of silver are formed at periodic intervals in a fine grain photographic emulsion containing silver halide when light of any given frequency is passed through the emulsion to a reflecting surface which directs the light back through the emulsion so as to form standing waves therein. Layers of latent silver are formed at the antinodes of the standing wave so that reflecting surfaces are obtained at periodic intervals throughout the depth of the emulsion. After exposing the emulsion to monochromatic light it is developed and treated with a fixer which dissolves unexposed silver halide. Shrinkage of the emulsion takes place in proportion to the amount of unexposed silver halide dissolved by the fixer, and the spacings between the deposited layers of silver vary directly with the shrinkage. When a transparent film containing reflecting layers at periodic intervals is subjected to light, a coherent reflection is obtained only for light at a frequency which is directly proportional to the spacing of the reflecting layers.

It has been discovered that a photographic emulsion containing silver halide may be subjected to light of a given frequency and at an intensity which varies from one point to another for forming an improved light filter. The amount of silver deposited in layers between the points varies as a function of the light intensity. During the fixing process, the amount of silver halide dissolved between the points varies inversely with the amount of silver deposited. Shrinkage of the emulsion increases, therefore, in a straight line ratio with the amount of silver halide dissolved. The final result is a film having thin layers of silver which are spaced from each other distances decreasing progressively between the points in the direction of decreasing light intensity. Such film has vari ous uses to which it may be put. To determine the unknown frequency of a beam of light, the beam may be directed against different points on the film until a coherent reflection of light is obtained. With the film calibrated to light frequencies, it is only necessary to note the point from which light is coherently reflected. If a light of particular frequency is desired, the film may be moved under a beam of white light until a coherent reflection of light at the desired frequency is obtained.

An object of this invention is to provide an improved light filter.

Another object is to provide a transparent film having light reflecting layers periodically spaced from each other distances varying in a straight line ratio between two laterally spaced points.

Still another object is to provide an improved device which is operable to produce a coherent reflection of light at frequencies varying progressively between two laterally spaced points thereon.

Yet another object is to provide an apparatus and method by which there may be produced a film having reflecting layers spaced from each other distances varying progressively between two points.

Theforegoing and other objects, features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 shows apparatus for subjecting a photographic emulsion to light so as to form reflecting layers therein spaced from each other distances varying progressively between its ends.

FIG. 2 is an enlarged side elevational view of a film formed by the apparatus of FIG. 1.

FIG. 3 is a graph showing how the coherent reflection of light from the film of FIG. 2 changes at different points along its length.

FIG. 4 shows a section of the film of FIG. 2 used for generating a light beam at some desirable frequency from a beam of white light.

FIG. 5 shows a section of the film used for determining the unknown frequency of a light beam.

FIG. 6 shows a calibrated film mounted for movement under a light beam.

Referring to FIG. 1 of the drawings, it will be noted that there is shown a photographic emulsion 2 uniformly coated upon a plate 4 which provides a reflecting surface upon its side adjacent the emulsion. Contained within the emulsion is silver halide from which latent silver is deposited in layers at periodic intervals throughout the depth of the emulsion when standing light waves are set up therein. After exposing the emulsion to light, it is developed and treated with a fixer which dissolves all of the remaining silver halide. This technique is sometimes used in color photography and the resulting film is generally referred to as a Lippmann film due to the early work by Gabriel Lippmann in this field.

When a film formed in the above manner is later subjected to light, the thin layers of deposited silver effect a coherent reflective scattering of light only when it is at a particular frequency for a given spacing between the layers. The amount of silver deposited in layers during the exposure of the emulsion to light varies as a function of the light intensity. When unexposed silver halide is dissolved by the fixer during processing, the emulsion shrinks in different areas by amounts which vary inversely with their exposure to light. The spacings between the reflecting layers decrease as the shrinkage increases and, therefore, are spaced at distances varying as a function of light exposure.

To produce an improved light filter from the emulsion 2, light is directed from a source 6 through a collimating lens 8 to a narrow band filter 10 which passes only light of a given frequency. Arranged between the filter 10 and the emulsion 2 is a neutral density filter 12 which passes amounts of light varying from one edge of the light beam to the other in such a manner that latent silver is deposited in a straight line ratio from silver halide in the emulsion. After exposing the emulsion to light of varying intensity from the filter 12, the reflecting surface is removed so the emulsion can be developed and treated with a fixer in a well known manner to provide a film 14 like that shown in FIG. 2. This film comprises layers 16 of transparent gelatin having thin layers 18 of silver deposited between them. As shown in FIG. 2, the film decreases in thickness from its left end toward its right end and therefore was subjected to light exposure which was greatest at the left end and decreased progressively toward the right. The amounts of silver halide remaining after exposure increased from left to right and its removal by the fixer caused an increase in shrinkage in the same direction.

During the process of development, the film is treated with mercuric chloride for partially reducing the silver in the layers to silver chloride. This causes the silver layers to become semitransparent so that part of any light directed against the top surface of the film passes through each of the layers. When light directed at any point on the surface of the film is at such a frequency that its wave length is twice the distance between adjacent reflecting layers at such point, then a coherent reflective scattering of light is obtained. For any other frequency the light is reflected incoherently from each of the reflecting layers, resulting in a considerably reduced intensity relative to the coherently reflected light. The frequency of the light which is coherently reflected from diflerent points on the film is indicated by the graph of FIG. 3. When using the film as either a frequency generator or as a device for determining the unknown frequency of a light beam, the frequency at which light is coherently reflected may be marked upon the edge of the film at uniformly spaced points.

In FIG. 4 an enlarged section of a film 14 is shown in use as a frequency generator. A beam 20 of white light is directed from a suitable source 22 to a converging lens 24 from which it passes through ,a small aperture in a plate 26 to a lens 28 that focuses the light to a spot on the film. The portion of this light at a frequency corresponding to the spacing of the reflective surfaces at the point where the light beam engages the film is coherently reflected as a beam 30 to a lens 32 from which a collimated beam of light passes to a point of use. By moving the film horizontally in either direction, any desired frequency of reflected light may be obtained.

For determining the unknown frequency of a light beam 36 in FIG. 5, the beam is directed through beam shaping devices to the upper surface of the film 14 while the latter is moved longitudinally until a coherently reflected light beam 40 is obtained. The beam 40 is focused by a lens 42 upon a photosensitive device 44 which produces a signal when a coherently reflected light beam has been detected. A reading may then be taken from an index on the film to determine the frequency of the light.

In FIG. 6 the film 14 is shown extending between two rollers 46 having knobs 48 by which they may be rotated to position the film under the light beam of either FIG. 4 or FIG. 5. An index 50 is provided along one edge of the film to indicate the frequency at which light is reflected from the film.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A device for generating a beam of light at a desired frequency from a beam of white light comprising, in combination,

a light sensitive transparent gelatin block having plural convergent layers of transparent gelatin separated one from another by plural partially reflecting planar 4 layers having spacings between reflecting layers progressively varying from one edge of said block to another as defined by said convergent gelatin layers, index markings at one edge of said block for indicating the frequency of light coherently reflected from the block at the different points,

means for directing a beam of white light at a point upon said block,

and means for moving said film longitudinally until said beam of white light acts thereon at a point corresponding to the frequency desired.

2. The device of claim 1 in which the spacings between the light reflecting layers decrease in a straight line ratio from one end of said film to the other.

3. A device for determining the unknown frequency of a beam of light comprising, in combination,

a light sensitive transparent gelatin block having plural convergent layers of transparent gelatin separated one from another by plural partially reflecting planar layers having spacings between reflecting layers progressively varying from one edge of said block to another as defined by said convergent gelatin layers,

means for directing at a point upon said block a light beam whose frequency is to be determined,

means for detecting light coherently reflected from said block at the point upon which said light beam of unknown frequency is directed,

means for moving said block longitudinally until said detecting means indicates that coherently reflected light has been detected,

and index markings at one edge of said block for indicating the frequency of light coherently reflected at different points.

4. An interference filter formed in a composite light sensitive transparent gelatin block, comprising, plural convergent layers of transparent gelating separated one from another by plural partially reflecting planar layers having spacings between reflecting layers progressively varying from one edge of said block to another as defined by said convergent gelatin layers.

5. The filter of claim 4, wherein said partially reflecting layers contain photographically developed latent silver and the spacings between such reflecting layers vary in a straight line ratio from one edge of the layers to another.

References Cited UNITED STATES PATENTS 2,432,950 12/1947 Turner et al. 350--314 X 2,590,906 4/1952 Tripp 350-166 2,952,188 9/1960 Bank 350314 3,088,227 5/1963 Tudor 3,217,591 11/1965 Barr et al. 356112 OTHER REFERENCES Neblett, C. B.: Photography: Its Materials & Processes D. Van Nostrand Co. Inc., New York, 1952, pp. 410-412.

The Construction of Interference Filters For the Transmission of Specified Wavelengths, K. M. Greenland et al.; Le Journal de Physique et le Radium; Tome II, Julliet 1950, p. 418.

Films Built by Depositing Successive Monomolecular Layers on a Solid Surface. Katharine Bladgett; Journal of The American Chemical Society; vol. 57; Issue 6; June 1935; p. 1007-1022.

RONALD L. WIBERT, Primary Examiner C. CLARK, Assistant Examiner 

